Correction apparatus for correcting the planarity of a metal strip and related correction method

ABSTRACT

A correction apparatus for correcting the planarity of a metal strip advancing into a coating system for coating said metal strip with a layer of molten metal, said apparatus comprising 
     a bridle roll, which modifies the feeding direction of the metal strip exiting an annealing furnace; 
     only one sink roll, arranged downstream of said bridle roll and inside a tank adapted to contain a bath of said molten metal; 
     a correction device for correcting the planarity of the metal strip arranged between said bridle roll and said sink roll; 
     wherein said correction device comprises at least two motorized rolls; 
     wherein a first motorized roll of said at least two motorized rolls is distal from said bridle roll in a fixed position with respect to a feeding path of the metal strip, said first motorized roll being arranged on a first side of said feeding path; 
     wherein said bridle roll is also arranged on said first side; 
     and wherein a second motorized roll of said at least two motorized rolls is proximal to said bridle roll and movable along the direction of the strip thickness, said second motorized roll being arranged on a second side, opposite to the first side, of said feeding path.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to PCT International Application No. PCT/IB2021/055965 filed on Jul. 2, 2021, which application claims priority to Italian Patent Application No. 102020000016012 filed on Jul. 2, 2020, the entire disclosures of which are expressly incorporated herein by reference.

STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT

Not Applicable

BACKGROUND Field of the Invention

The present invention relates to the field of systems for coating flat products made of a ferromagnetic material, e.g. metal strips, in particular strips made of steel. More specifically, the invention relates to a correction apparatus, and to a related correction method, for correcting the planarity of a strip made of a ferromagnetic material moving during a molten metal coating process, e.g. a galvanizing process. The present invention further relates to a system for coating a metal strip with molten metal comprising such a correction apparatus.

Background Art

As known, strips made of a ferromagnetic material are externally coated by means of a plurality of coating processes, e.g. by galvanizing.

The zone of the tank containing the bath of molten metal, e.g. zinc, is the heart of the coating process and affects system operation, process productivity, product quality, and zinc consumption.

In such coating processes, the moving metal strip is normally subject to deformations and vibrations, which are corrected by using some devices in order to improve the process productivity and optimize the zinc consumption.

A known example of a system for coating a metal strip is shown in FIG. 1 . Such a system comprises:

-   -   an annealing furnace 10, which is crossed by the metal strip 1;     -   at least one bridle roll 2, which modifies the direction of the         feeding path of the metal strip 1 exiting the annealing furnace         10 and applies a tension to the strip;     -   a tank 11 containing the molten metal bath 12;     -   a connection duct 17 between the annealing furnace 10 and the         tank 11;     -   three idle rolls 3, 60, 50, with corresponding bearings,         immersed in the molten metal bath 12;     -   immersed support arms (not shown), supporting the respective         idle rolls 3, 60, 50;     -   a pair of air knives 15, arranged downstream of the tank 11         along the strip feeding path, to remove excess molten metal from         the strip.

The sink deflector roll 3, which is proximal to the connection duct 17 considering the strip feeding path, deflects the strip exiting the annealing furnace 10 from an oblique position to a vertical position, defining the position of the air knives 15.

The other two sink rolls 60, 50, known as stabilizer rolls and having a smaller diameter than the sink deflector roll 3, are positioned on the vertical stretch of the strip feeding path, thus in series with respect to said sink deflector roll 3, and define a correction device for correcting the planarity of the strip.

In particular, the sink stabilizer roll 50 is distal from the sink deflector roll 3 and in a fixed position with respect to the feeding path of the metal strip 1. The sink stabilizer roll 50 is arranged on a first side of the feeding path, which is the same side on which the sink deflector roll 3 is arranged.

The sink stabilizer roll 60 is instead proximal to the sink deflector roll 3 and is positionable along a horizontal direction, transverse to the feeding path of the metal strip, so as to bend it in the opposite direction to the bending imparted by the sink deflector roll 3. In fact, the sink stabilizer roll 60 is arranged on a second side, opposite to the first side, of the feeding path, and is capable of performing a penetration with respect to the theoretical strip passage plane.

Disadvantageously, as shown in FIG. 6 , this allows attenuating only partially the typical defect of the crossbow shape (or crossbow effect), which is obtained once the strip has bent about the sink deflector roll 3.

Furthermore, the duration over time of the sink rolls is about two or three weeks. The main causes imposing the replacement of the sink rolls are:

-   -   the wear of the surfaces of all sink rolls in contact with the         strip;     -   the presence of slag in the coating metal on the surface of the         rolls which can mark the strip;     -   the wear of the bearings of the two sink stabilizer rolls;     -   the locking of the rotation of the sink stabilizer rolls with         the risk of scratching the surface of the strip;     -   the difficulty of rotation of the sink stabilizer rolls at high         speed when thin strips need to be coated;     -   the wear of the bearings of the sink deflector roll;     -   the vibrations generated by the wear of the sink deflector roll         and/or the bearings of said deflector roll.

Stopping the system to replace the several sink rolls in the tank, in addition to generating a large amount of waste material because part of the material is not processed correctly, results in a production loss and increased production costs of the system due to the energy consumption for the annealing furnace and the tank during the stop.

The need is thus felt to manufacture an innovative correction apparatus for correcting the planarity of the strip which allows the aforesaid drawbacks to be overcome.

SUMMARY OF THE INVENTION

It is the object of the present invention to manufacture a correction apparatus for correcting the planarity of a metal strip, advancing into a coating system, which is capable of reducing as much as possible, until almost eliminating, the crossbow effect of the strip caused by the combined effect of winding the strip about the sink deflector roll and applying a pull to said strip.

It is another object of the invention to manufacture a correction apparatus which allows reducing the number of sink rolls, and therefore reducing the stops of the system due to the replacement of said sink rolls, thus minimizing the production losses and energy costs associated with these stops.

It is another object of the invention to carry out a related method for correcting the planarity of the metal strip.

Therefore, the present invention aims to achieve the above-discussed objects by manufacturing a correction apparatus for correcting the planarity of a metal strip advancing into a coating system for coating said metal strip with a layer of molten metal, said apparatus comprising

-   -   a bridle roll adapted to modify the direction of the feeding         path of the metal strip exiting an annealing furnace;     -   only one sink roll, arranged downstream of the bridle roll along         said feeding path and inside a tank adapted to contain a bath of         said molten metal, said sink roll being adapted to modify the         direction of the feeding path of the metal strip immersed in the         tank;     -   a correction device for correcting the planarity of the metal         strip, arranged between said bridle roll and said sink roll;

wherein said correction device comprises at least two motorized rolls arranged outside the tank,

where a first motorized roll of said at least two motorized rolls is distal from said bridle roll, in a fixed position with respect to the metal strip feeding path, said first motorized roll being arranged on a first side of said feeding path;

where said bridle roll is also arranged on said first side;

and wherein a second motorized roll of said at least two motorized rolls is arranged on a second side, opposite to the first side, of said feeding path, is proximal to said bridle roll and movable along a trajectory which intersects a plane A, defined by the rotation axes X, Y of the bridle roll and the first motorized roll, respectively, in an intermediate position between said rotation axes X, Y, so as to bend the metal strip in the opposite direction to the bending imparted by said bridle roll.

Another aspect of the invention relates to a method for correcting the planarity of a metal strip advancing into a coating system for coating said metal strip with a layer of molten metal, the method being carried out by the aforesaid correction apparatus and comprising the following steps:

a) modifying, by means of the bridle roll, the direction of the feeding path of the metal strip exiting an annealing furnace;

b) modifying again, by means of the sink roll, the direction of the feeding path of the metal strip immersed in the tank;

wherein the step of correcting the planarity of the metal strip by means of the at least two motorized rolls of the correction device is provided between step a) and step b), by moving the second motorized roll along a trajectory which intersects the plane A, defined by the rotation axes X, Y of the bridle roll and the first motorized roll, respectively, in an intermediate position between said rotation axes X, Y, so as to bend said metal strip in the opposite direction to the bending imparted by said bridle roll.

Advantageously, the correction device is installed outside the molten metal tank, preferably immediately after the hot roll of the annealing furnace and before a connection duct of the furnace with the tank of the molten metal bath. Alternatively, the correction apparatus of the invention is installed downstream of the hot roll of the annealing furnace, but at least partially inside the connection duct of the furnace with the tank of the molten metal bath.

Therefore, the sink stabilizer rolls in the tank are no longer necessary with an apparent simplification of the system, and thus apparent benefits in terms of reducing the Capex and Opex.

The provision of the device for correcting the planarity of the strip equipped with at least two motorized rolls, of which one movable along the aforesaid trajectory which intersects the theoretical strip feeding plane, and of the fixed bridle roll on said theoretical feeding plane, allows correcting, in a more efficient manner, the crossbow effect of the coated strip, caused by the combined effect of winding the strip about the sink roll and applying a pull to said strip.

In particular, as shown in FIG. 7 , the combination of a first bending imparted by the second motorized roll of the correction device, outside and upstream of the tank 11, with a second bending imparted by the sink deflector roll 3, in the opposite direction to the first bending, improves the correction of the crossbow effect exiting the tank 11 as compared to the solutions of the prior art (FIG. 6 ).

A further improvement in the planarity of the strip is obtained by providing a correction device with three motorized rolls, of which the central motorized roll is movable along the trajectory which intersects the plane A, defined by the rotation axes X, Y of the bridle roll and the first motorized roll, respectively, in an intermediate position between said rotation axes X, Y, while the two lateral motorized rolls are fixed on said theoretical feeding plane. The two lateral motorized rolls are arranged on the same side as the bridle roll with respect to the path of the strip, while the central motorized roll is arranged on the opposite side.

Preferably, due to the aid of a measurement device for measuring the planarity of the metal strip, arranged downstream of the tank along the feeding path and communicating with a control system, which, in turn, can act in feedback on the second motorized roll to adjust the position thereof along the aforesaid trajectory, it is possible to adjust the planarity correction in case of the presence of residual errors downstream of the tank.

A further improvement in the correction of the planarity of the strip coated with molten metal can be obtained by providing, downstream of the tank, along the strip feeding path, at least one pair of electromagnetic stabilizer devices. At least one further measurement device for measuring the planarity of the metal strip, for example comprising position transducers based on the eddy currents or at least one laser scanning system, can be arranged close to said stabilizer devices and communicating with the aforesaid control system, which can thus further act in feedback on the second motorized roll to adjust again the position thereof along said trajectory, allowing the optimization of the planarity correction.

Other advantages of the invention are:

-   -   less wear of the bearings of the motorized rolls of the         correction device, these rolls not being immersed in the metal         bath;     -   removal of the risk of locking the rotation of the rolls of the         correction device, precisely because of not being immersed, thus         reducing the risk of scratches on the surface of the strip;     -   reduction in marks on the strip due to the presence of slag of         the coating metal exclusively on the surface of the only sink         roll;     -   improved rotation of the rolls of the correction device at high         speed when thin strips need to be coated, because these rolls         are motorized and not immersed in the bath;     -   reduction in waste material due to the reduction of the system         stops.

The dependent claims describe preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become more apparent in the light of the detailed description of preferred, but not exclusive, embodiments of an apparatus shown by way of non-limiting example, with the aid of the accompanying drawings, in which:

FIG. 1 is a diagram of a strip coating system with a correction apparatus for correcting the planarity of the strip according to the prior art;

FIG. 2 is a diagram of a strip coating system with a first embodiment of the correction apparatus for correcting the planarity of the strip according to the invention, in a first operating position;

FIG. 3 is the diagram in FIG. 2 with the correction apparatus in a second operating position;

FIG. 4 is a diagram of a strip coating system with a second embodiment of the correction apparatus for correcting the planarity of the strip according to the invention, in a first operating position;

FIG. 5 is the diagram in FIG. 4 with the correction apparatus in a second operating position;

FIG. 6 diagrammatically depicts the planarity of a strip at different zones of an apparatus of the prior art;

FIG. 7 diagrammatically depicts the planarity of a strip at different zones of an apparatus according to the invention.

The same reference numerals in the figures identify the same elements or components.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

With reference to FIGS. 2 to 5 , exemplary embodiments of a system for coating a metal strip 1 with a layer of molten metal are shown, the system comprising a correction apparatus according to the invention, which is suitable for correcting the planarity of the metal strip 1 advancing along said system. As known, the metal strip is a product having a dimension, i.e., the thickness, which is significantly smaller than the other two dimensions, i.e., the length and width.

In all embodiments of the invention, such a correction apparatus comprises

-   -   at least one bridle roll 2, which modifies the direction of the         feeding path of the metal strip 1 exiting an annealing furnace         10 and which can apply a tension to the strip;     -   only one sink roll 3, arranged downstream of the bridle roll 2         along the feeding path and inside a tank 11 adapted to contain a         bath 12 of molten metal, said sink roll 3 modifying the         direction of the feeding path of the metal strip 1 immersed in         the tank 11;     -   a correction device 4 for correcting the planarity of the metal         strip, arranged between the bridle roll 2 and the sink roll 3.

Advantageously, only one correction device 4 is provided, arranged between the bridle roll 2 and the sink roll 3. In particular, according to the invention, the presence of further correction devices is excluded; in particular, the presence of a further correction device comprising sink rolls inside the tank 11 and/or other rolls arranged downstream of said tank 11 is excluded.

Two bridle rolls 2, 2′ are shown in FIGS. 2 to 5 and the correction device 4 is arranged between the last bridle roll 2 and the sink roll 3. At least two bridle rolls can be provided, e.g., three or more bridle rolls. Such bridle rolls can be motorized or idle, e.g., some being motorized and others idle.

Advantageously, the correction device 4 comprises at least two motorized rolls 5, 6 arranged outside the tank 11, therefore never immersed in the molten metal, e.g., zinc. Said at least two motorized rolls 5, 6 are not bridle rolls.

A first motorized roll 5 of said at least two motorized rolls is distal from the bridle roll 2 and in a fixed position with respect to the feeding path of the metal strip 1, said first motorized roll 5 being arranged on a first side of said feeding path. The bridle roll 2 is also arranged on said first side. A second motorized roll 6 of said at least two motorized rolls is arranged on a second side, opposite to the first side, of the feeding path, is proximal to the bridle roll 2 and movable along a trajectory which intersects a plane A, defined by the rotation axes X, Y of the bridle roll 2 and the first motorized roll 5, respectively, in an intermediate position between said rotation axes X, Y so as to bend the metal strip in the opposite direction to the bending imparted by the bridle roll 2 (FIGS. 3 and 5 ). The second motorized roll 6, moving along the aforesaid trajectory in the space between the bridle roll 2 and the fixed motorized roll 5, thus performs a penetration on the strip, which generates a localized plastic deformation opposite to that which will then be produced by the sink roll 3.

In a variant, the longitudinal axis of the second motorized roll 6 can move along a transverse plane, e.g. perpendicular, to the theoretical strip feeding plane, and which intersects said theoretical strip feeding plane between the bridle roll 2 and the fixed motorized roll 5.

Instead, in a further variant, the longitudinal axis of the second motorized roll 6 can move along a curved surface which intersects the theoretical strip feeding plane between the bridle roll 2 and the fixed motorized roll 5.

In particular, in both variants, the longitudinal axis of the second motorized roll 6 is parallel to said theoretical feeding plane and can move with respect to the latter along said transverse plane or said curved surface in a first direction or in a second direction, opposite to the first direction, so as to adjust the aforesaid localized plastic deformation, the second motorized roll 6 more or less penetrating said theoretical plane, e.g., along the direction of the strip thickness.

In the zone between the bridle roll 2 and the sink roll 3, theoretical strip feeding plane means a tangent plane, in common with the bridle roll 2 and the sink roll 3, which is tangent in the zone where the strip leaves the bridle roll 2 and in the zone where the strip starts being wound onto the sink roll 3, respectively.

Since the sink roll 3 is the only roll immersed in the molten metal of the tank 11, the support arms (not shown) of the sink roll 3, which is preferably idle, are the only support arms partially immersed in the molten metal.

Preferably, the tank 11 is a tank internally coated with a ceramic material and internally provided with heaters (not shown), e.g., heaters based on the magnetic field obtained with suitable inductors, to melt the metal ingots, e.g., zinc, and keep the molten metal moving and at temperature.

In a first embodiment of the apparatus of the invention, shown in FIGS. 2 and 3 , only two motorized rolls 5,6 are provided. Advantageously, in this case, the penetration on the strip, obtained by the movement of the second motorized roll 6 along the trajectory which intersects the plane A, is obtained between the bridle roll 2, i.e., the last roll of the so-called hot roll provided inside the casing of the annealing furnace 10, and the first motorized roll 5, both arranged on the same side with respect to the feeding path, therefore on the same side with respect to the theoretical strip feeding plane in the zone between the bridle roll 2 and the sink roll 3.

In a second embodiment of the apparatus of the invention, shown in FIGS. 4 and 5 , the correction device 4 also comprises a third motorized roll 7 arranged between the bridle roll 2 and the second motorized roll 6, in a fixed position with respect to the feeding path of the metal strip 1. This third motorized roll 7 is arranged on said first side of the feeding path. Advantageously, in this case, the penetration on the strip, obtained by the movement of the second motorized roll 6 along the trajectory which intersects the plane A, is obtained between the third motorized roll 7 and the first motorized roll 5, both arranged on the same side with respect to the feeding path, therefore on the same side with respect to the theoretical strip feeding plane in the zone between the bridle roll 2 and the sink roll 3. This solution avoids modifying the winding angle of the strip leaving the bridle roll 2 when the second motorized roll 6 is moved along said trajectory.

In both embodiments, there can be provided

-   -   at least one measurement device 13 for measuring the planarity         of the metal strip 1, arranged downstream of the tank 11 along         the feeding path;     -   and a control system 14 adapted to acquire planarity data from         the measurement device 13 and send a control signal to the         correction device 4 to possibly adjust the position of the         second motorized roll 6 along the aforesaid trajectory.

The measurement device 13 can comprise, or consist of, at least one laser scanning system.

At least one pair of air knives 15, e.g., only one pair of air knives, can be arranged downstream of the tank 11 along the feeding path. A first air knife is arranged at the first side of the feeding path and a second air knife is arranged at the second side of said feeding path. The air knives remove excess molten metal from the metal strip, thus allowing the coating thickness provided by the specifications of the coated product to be obtained.

The measurement device 13 is arranged upstream or downstream of the at least one pair of air knives 15 along the feeding path.

Preferably, there is provided at least one pair, e.g., only one pair, of electromagnetic stabilizer devices 16, arranged downstream of the tank 11 along the feeding path. A first electromagnetic stabilizer device is arranged at the first side of the feeding path and a second electromagnetic stabilizer device is arranged at the second side of said feeding path. This electromagnetic damping system has the dual purpose of removing or controlling the vibration of the strip, if the vibration level produced by the bearings of the sink roll 3 is not acceptable, and optionally correcting a residual planarity error of the strip profile.

If the electromagnetic stabilizer devices 16 are provided, the control system 14 is adapted to acquire further planarity data, from at least one further measurement device 13′ associated with the at least one pair of electromagnetic stabilizer devices 16, and adapted to send a further control signal to the correction device 4 to possibly adjust the position of the second motorized roll 6 along the aforesaid trajectory.

At least one pair of electromagnetic stabilizer devices 16 can be arranged downstream of the at least one pair of air knives 15 along the metal strip feeding path.

Therefore, the measurement device 13 close to the air knives 15 and the measurement device 13′ close to the optional electromagnetic stabilizer devices 16 are connected, by means of respective interfaces, to the control system 14 which automatically adjusts the position of the second motorized roll 6 along the trajectory intersecting plane A.

Preferably, the correction device 4 comprises a movement mechanism for moving the second motorized roll 6 along said trajectory. Therefore, the control system 14 sends the control signals to said movement mechanism. For example, such a movement mechanism can be selected from the following: pneumatic, hydraulic, mechanical, electromechanical actuators, or the like.

In the embodiments shown in the Figures, the bridle roll 2 and the two or three motorized rolls 5,6,7 are preferably arranged inside an end part of the annealing furnace 10, preferably upstream of a connection duct 17, which connects said end part of the annealing furnace 10 to the tank 11.

In a variant, the two or three motorized rolls 5, 6, 7 are arranged, at least partially, inside the connection duct 17.

In all embodiments of the invention, the two or three motorized rolls 5, 6, 7 are preferably coated with tungsten carbide or another suitable ceramic coating, which ensures a high wear resistance thereof.

The two or three rolls 5, 6, 7 are motorized in order to avoid any possible slippage with respect to the advancing strip.

The position of the second motorized roll 6 is detected by a convenient position transducer, or another suitable position sensor, and the signal containing the data of this position is sent to the control system 14.

In all embodiments of the invention, the sink roll 3 has a diameter D3 which is much greater than the diameter D1 of the two or three motorized rolls 5, 6, 7 of the correction device 4. The diameter D2 of the bridle roll 2 can have a value which is greater than or equal to D3 or can have an intermediate value between D1 and D3.

For example, in a first variant, the diameter D3 of the sink roll 3 can be between 600 and 1000 mm; the diameter D2 of the bridle roll 2 can be between 600 and 1200 mm; and the diameter D1 of the two or three motorized rolls 5, 6, 7 can be between 150 and 400 mm.

Preferably, the ratio D3/D1 is from 1.5 to 6.6, the ratio D3/D2 is from 0.5 to 1.6, and the ratio D2/D1 is from 1.5 to 8.

For example, instead, in a second variant, the diameter D3 of the sink roll 3 can be between 600 and 1000 mm; the diameter D2 of the bridle roll 2 can be between 400 and 900 mm; and the diameter D1 of the two or three motorized rolls 5, 6, 7 can be between 150 and 400 mm.

Preferably, the ratio D3/D1 is from 1.5 to 6.6, the ratio D3/D2 is from 0.6 to 2.5, and the ratio D2/D1 is from 1 to 6.

In alternative variants, the motorized roll 5 and the optional motorized roll 7 can have a different diameter than the diameter D1 of the motorized roll 6.

Preferably, with reference to the second embodiment, the distance along the strip feeding plane between the motorized rolls 5 and 7 is between 1.05 D1 and 2 D1, preferably between 1.5 D1 and 2 D1 (where, for example, 1.05 D1 means 1.05 times D1, and so on). The motorized roll 6, on the opposite side of the strip with respect to rolls 5, 7, is arranged within this distance.

Preferably, with reference to the first embodiment, the distance along the feeding path of the metal strip 1 between the first motorized roll 5 and the bridle roll 2 is between 1.05 D2 and 2 D2, preferably between 1.5 D2 and 2 D2, said second motorized roll 6 being arranged within this distance.

The sink roll 3, which is preferably idle, is preferably coated with tungsten carbide or another suitable ceramic coating, which ensures a high wear resistance thereof.

The bearings allowing the idle sink roll 3 to roll are designed so as to ensure a service life of more than three months of continuous working, in order to limit the vibrations generated by the sink roll 3 due to the wear of the bearings. Such bearings can be of the rolling type, for example made of metals or ceramics which are resistant to wear caused by the mechanical and chemical stresses of the tank.

A method for correcting the planarity of a metal strip 1 advancing into a coating system, such as that shown in FIGS. 2 to 5 , is described below.

Such a method comprises the following steps:

-   -   a) modifying, by means of the bridle roll 2, the direction of         the feeding path of the metal strip 1 exiting the annealing         furnace 10, optionally applying a tension to the strip;     -   b) modifying again, by means of the sink roll 3, the direction         of the feeding path of the metal strip 1 immersed in the tank         11;

wherein the correction of the planarity of the metal strip by means of the at least two motorized rolls 5, 6 of the correction device 4 is provided between step a) and step b), preferably only between step a) and step b), by moving the second motorized roll 6 along a trajectory which intersects the plane A, defined by the rotation axes X, Y of the bridle roll 2 and the first motorized roll 5, respectively, in an intermediate position between said rotation axes X, Y, so as to bend the metal strip in the opposite direction to the bending imparted by the bridle roll 2.

The sink roll 3 deflects the strip exiting the annealing furnace 10, in particular exiting the connection duct 17, from an oblique position with respect to the vertical to a substantially vertical position that defines the position of the possible air knives 15.

Preferably, in order to correct the planarity of the metal strip, the following steps are included:

-   -   measuring the planarity of the metal strip 1 by means of the         measurement device 13 arranged downstream of the tank 11 along         the feeding path;     -   acquiring planarity data from the measurement device 13 and         sending a control signal, by means of the control system 14, to         the correction device 4 to possibly adjust the position of the         second motorized roll 6 along said trajectory.

The measurement of the planarity of the metal strip 1 can be carried out upstream or downstream of the at least one pair of air knives 15. At the outlet of the tank 11, the profile and therefore the planarity of the strip can be measured or detected by a suitable scanning system based on laser technology. The value of the static shape defects, such as the crossbow, present on the strip is then sent to the control system 14 for a possible feedback on the second motorized roll 6.

Furthermore, if the electromagnetic stabilizer devices 16 for correcting the planarity of the metal strip are provided, the following steps are also included:

-   -   acquiring planarity data also from a further measurement device         13′ associated with at least one pair of electromagnetic         stabilizer devices 16, arranged downstream of the tank 11, and         sending a control signal, by means of the control system 14, to         the correction device 4 to possibly adjust the position of the         second motorized roll 6 along the aforesaid trajectory.

The electromagnetic stabilizer devices 16 allow reducing the amplitude of the vibration of the strip in the zone of the air knives 15 and simultaneously improving the shape of the strip by reducing the amplitude of the static shape defects, such as the crossbow. 

1. A correction apparatus for correcting a planarity of a metal strip advancing into a coating system for coating said metal strip with a layer of molten metal, said apparatus comprising a bridle roll adapted to modify a direction of a feeding path of the metal strip exiting an annealing furnace; only one sink roll, arranged downstream of the bridle roll along said feeding path and inside a tank adapted to contain a bath of said molten metal, said sink roll being adapted to modify the direction of the feeding path of the metal strip inside the tank; a correction device for correcting the planarity of the metal strip, arranged between said bridle roll and said sink roll; wherein said correction device comprises at least two motorized rolls arranged outside the tank, wherein a first motorized roll of said at least two motorized rolls is distal from said bridle roll in a fixed position with respect to the feeding path of the metal strip, said first motorized roll being arranged on a first side of said feeding path; wherein said bridle roll is also arranged on said first side; and wherein a second motorized roll of said at least two motorized rolls is arranged on a second side, opposite to the first side, of said feeding path, is proximal to said bridle roll and movable along a trajectory which intersects a plane A, defined by rotation axes X, Y of the bridle roll and the first motorized roll, respectively, in an intermediate position between said rotation axes X, Y, so as to bend said metal strip in an opposite direction to a bending imparted by said bridle roll.
 2. The apparatus according to claim 1, wherein said correction device comprises a third motorized roll arranged between said bridle roll and said second motorized roll in a fixed position with respect to the feeding path of the metal strip, said third motorized roll being arranged on said first side of the feeding path.
 3. The apparatus according to claim 1, wherein there are provided at least one first measurement device for measuring the planarity of the metal strip, arranged downstream of the tank along the feeding path; and a control system adapted to acquire planarity data from said at least one first measurement device and send a control signal to said correction device to possibly adjust a position of the second motorized roll along said trajectory; preferably, wherein at least one position sensor is provided for detecting the position of the second motorized roll, said at least one position sensor being adapted to send a signal containing the data of said position of the second motorized roll to the control system.
 4. The apparatus according to claim 3, wherein at least one pair of air knives is provided, arranged downstream of the tank along the feeding path, wherein a first air knife is arranged at the first side of the feeding path and a second air knife is arranged at the second side of said feeding path; and wherein said at least one first measurement device is arranged upstream or downstream of said at least one pair of air knives along the feeding path.
 5. The apparatus according to claim 3, wherein there is provided at least one pair of electromagnetic stabilizer devices, arranged downstream of the tank along the feeding path, and of which a first electromagnetic stabilizer device is arranged at said first side and a second electromagnetic stabilizer device is arranged at said second side; and wherein the control system is adapted to acquire further planarity data from at least one second measurement device associated with said at least one pair of electromagnetic stabilizer devices, and adapted to send a control signal to said correction device to possibly adjust the position of the second motorized roll along said trajectory.
 6. The apparatus according to claim 5, wherein said at least one pair of electromagnetic stabilizer devices is arranged downstream of at least one pair of air knives along the feeding path, said at least one pair of air knives being arranged downstream of the tank.
 7. The apparatus according to claim 3, wherein said first measurement device comprises a laser scanning system.
 8. The apparatus according to claim 1, wherein said correction device comprises a movement mechanism for moving said second motorized roll along said trajectory.
 9. The apparatus according to claim 1, wherein said bridle roll and said at least two motorized rolls are arranged inside an end part of the annealing furnace, preferably upstream of a connection duct which connects said end part to the tank.
 10. The apparatus according to claim 1, wherein, with D2 being a diameter of the bridle roll, a distance along the feeding path of the metal strip between the first motorized roll and said bridle roll is between 1.05 D2 and 2 D2, preferably between 1.5 D2 and 2 D2; said second motorized roll being arranged within said distance.
 11. The apparatus according to claim 2, wherein, with D1 being a diameter of the first motorized roll, the second motorized roll, and the third motorized roll, a distance along the feeding path of the metal strip between the first motorized roll and the third motorized roll is between 1.05 D1 and 2 D1, preferably between 1.5 D1 and 2 D1; said second motorized roll being arranged within said distance.
 12. The apparatus according to claim 1, wherein the sink roll has a diameter D3 which is greater than a diameter D1 of said at least two motorized rolls, and wherein a diameter D2 of the bridle roll has a value which is greater than or equal to the diameter D3 or said diameter D2 can have an intermediate value between the diameter D1 and the diameter D3.
 13. The apparatus according to claim 12, wherein the ratio D3/D1 is from 1.5 to 6.6, the ratio D3/D2 is from 0.5 to 1.6, and the ratio D2/D1 is from 1.5 to
 8. 14. The apparatus according to claim 13, wherein, in the case of the diameter D2 having a value which is greater than or equal to the diameter D3, the diameter D3 of the sink roll is in a range from 600 to 1000 mm; the diameter D2 of the bridle roll is in a range from 600 to 1200 mm; and the diameter D1 of said at least two motorized rolls is in a range from 150 to 400 mm.
 15. The apparatus according to claim 12, wherein the ratio D3/D1 is from 1.5 to 6.6, the ratio D3/D2 is from 0.6 to 2.5, and the ratio D2/D1 is from 1 to
 6. 16. The apparatus according to claim 13, wherein, in the case of the diameter D2 having an intermediate value between the diameter D1 and the diameter D3, the diameter D3 of the sink roll is in a range from 600 to 1000 mm; the diameter D2 of the bridle roll is in a range from 400 to 900 mm; and the diameter D1 of said at least two motorized rolls is in a range from 150 to 400 mm.
 17. The apparatus according to claim 1, wherein at least two bridle rolls are provided and only one correction device is arranged between the last bridle roll and said sink roll.
 18. A method for correcting a planarity of a metal strip advancing into a coating system for coating said metal strip with a layer of molten metal, the method being carried out by a correction apparatus according to claim 1 and comprising the following steps: a) modifying, by means of the bridle roll, the direction of the feeding path of the metal strip exiting an annealing furnace; b) modifying again, by means of the sink roll, the direction of the feeding path of the metal strip immersed in the tank; wherein the step of correcting the planarity of the metal strip by means of the at least two motorized rolls of the correction device is provided between step a) and step b), by moving the second motorized roll along a trajectory which intersects the plane A, defined by the rotation axes X, Y of the bridle roll and the first motorized roll, respectively, in an intermediate position between said rotation axes X, Y, so as to bend said metal strip in the opposite direction to the bending imparted by said bridle roll.
 19. The method according to claim 18, wherein in order to correct the planarity of the metal strip, the following steps are included: measuring the planarity of the metal strip by means of at least one first measurement device arranged downstream of the tank along the feeding path; acquiring planarity data from said first measurement device and sending a control signal, by means of a control system, to said correction device to possibly adjust a position of the second motorized roll along said trajectory.
 20. The method according to claim 19, wherein the step of measuring the planarity of the metal strip is carried out upstream or downstream of at least one pair of air knives.
 21. The method according to claim 19, wherein, in order to correct the planarity of the metal strip, the following steps are also included: acquiring further planarity data from at least one second measurement device associated with at least one pair of electromagnetic stabilizer devices, arranged downstream of the tank, and sending a further control signal, by means of the control system, to said correction device to possibly adjust the position of the second motorized roll along said trajectory.
 22. A system for coating a metal strip with a layer of molten metal, comprising a correction apparatus according to claim 1 for correcting the planarity of the metal strip advancing along said system. 